1. Field of the Invention
The invention relates to a composite for the absorption of energy, comprising a matrix of embedded fibers.
The composites are used, for example, in motor vehicles for the mounting of fenders or other parts which are needed to convert kinetic energy into deformation energy in a collision. Composites are known in which glass fibers or carbon fibers are embedded in a matrix of epoxy resin. Although such composites, in particular those with carbon fibers, can absorb a relatively high energy, they exhibit an uncontrolled behavior and are so brittle that they splinter in a collision and the mechanical connection is broken.
The present invention is based on the object of creating a composite which exhibits a controlled behavior, has a high energy absorption capacity, and does not splinter but retains structural continuity.
This object of the invention is achieved by using fibers having a breaking length of at least 80 km and an elongation at break of at least 2% embedded in a matrix at an angle of +/- (45.degree. . . . 90.degree.) to the direction of pressure application. The fibers preferably consist of a linear polyethylene (PE) of ultrahigh molecular weight, M&gt;10 g/mol.
A process for the production of such fibers of a linear polyethylene of ultrahigh molecular weight is described, for example, in GB-A-Nos. 2042414 and 2051667. In this case, a polyolefin is dissolved in a solvent, the solution is deformed into fibers at a temperature above the solution temperature of the polYolefin, the fibers are cooled to a temperature below the solution temperature for gelation and the gelled fibers are stretched. Stretching takes place with solvent removal. This procedure yields fibers with extremely long and highly oriented molecules. Such fibers when used in composites within a matrix of a resin exhibit a controlled failure with high energy absorption, i.e., it can be predicted how and when the body deforms with static or dynamic application of the load.
The polyethylenes may contain minor amounts, preferably at most 5 mol%, of one or more other alkenes capable of copolymerization with them, such as propylene, butylene, hexene, 4-methylpentene, octene etc. and have 1 to 10, in particular 2 to 6 methyl or ethyl groups per 1000 carbon atoms.
Other polyolefins also can be used. For example, polypropylene homopolymers and copolymers can be used in the present invention. Furthermore, the polyolefins used may also contain minor amounts of one or more other polymers, in particular alkene-1 polymers.
In a further development of the invention, these fibers are embedded in a matrix with other, preferably inorganic, fibers. The other fibers form an acute angle from 0.degree. to +/- to the direction of pressure application.
According to a preferred embodiment of the present invention, a plurality of layers of the different fibers are arranged alternately. For example, fibers having a breaking length cf at least 80 km and an elongation at break of at least 2%, or PE fibers, may be arranged on the outsides. These fibers may surround other fibers, such as inorganic fibers. Thus, for example, if carbon fibers or glass fibers are used with preference as to other fibers, these fibers are prevented from breaking by the surrounding fibers having a breaking length of at least 80 km and an elongation at break of at least 2%, or by the PE fibers. In this manner, the high energy absorption properties of the carbon fibers or glass fibers are combined with the properties of the controlled-deforming first fiber.
According to a preferred embodiment, the proportion of the fibers is 40 to 85% by volume, the proportion of the fibers having a breaking length of at least 80 km and an elongation at break of at least 2%, or of the PE fibers, preferably being 10 to 85% by volume.
Preferably, the fibers having a breaking length of at least 80 km and an elongation at break of at least 2%, or the PE fibers, are arranged at an angle of +/- (45.degree. to 90.degree.), and the other fibers are arranged at angles of +/- (0.degree. to 30.degree.) to the direction of pressure application. The other fibers in this case preferably have a low elongation at break (&lt;5%) and consist, for example, of carbon fibers or glass fibers.
The matrix in which the fibers are embedded in layers preferably consists of a duroplastic or thermoplastic matrix system with a processing temperature of up to about 130.degree. C., e.g., an epoxy resin, a polyester or another material system that is compatible with the fibers having a breaking length of at least 80 km and an elongation at break of at least 2%, or with the PE fibers.
The composite preferably has the form of a hollow body of any cross-section perpendicular to the axis of pressure application. For example, the composite can be in the form of a tube or a box girder.
According to a further preferred embodiment, the composite is made as an integrally stiffened plate, such as a corrugated plate. In a corrugated plate, the pressure is applied parallel to the extent of the width of the corrugations. The corrugated plates are preferably fastened at their edges, so that a controlled buckling or a controlled folding takes place when a load or impact force is applied.
In a further development of the invention, the cross-section changes in the direction of the pressure application with respect to the cross-sectional area or the cross-sectional shape.
Such composites are suitable in particular for supporting frames of helicopters, for floors of helicopters, for mounting fenders, for aircraft seats or elements where the occurrence of impact energies is to be expected.
The fibers in the layers preferably take the form of roving, woven fabrics, knitted fabrics, ribbons or braided or woven hoses, including intralaminar fiber mixtures containing fibers having a breaking length of at least 80 km and an elongation at break of at least 2%, or the PE fibers.
According to a further preferred embodiment, for the reduction of initial load peaks, the laminate, the fastening or the supports designed at one or both ends of the composite as a fail initiator (trigger mechanism), which is structurally formed in such a way that it contributes to an energy-consuming snapping of the 90.degree.-oriented fibers having a break of at least 2%, or of the PE fibers.